1. Technical Field
The present invention relates to a polarization conversion element and a method for manufacturing a polarization conversion element that converts light vibrating in various directions into light vibrating in a single polarization direction.
2. Related Art
Polarization conversion elements are known as an optical element that converts light vibrating in various directions into light vibrating in a single polarization direction.
FIGS. 22A and 22B are schematic views for describing a related art common polarization conversion element. FIG. 22A is a perspective view of the polarization conversion element and FIG. 22B is a plan view of the polarization conversion element of FIG. 22A viewed from +z direction. Referring to FIGS. 22A and 22B, this polarization conversion element 50 includes a polarization separation element 51 and a half λ retardation plate 52. The polarization separation element 51 separates incident light into two types of polarized light. The half λ retardation plate 52 is selectively disposed on a light emitting side of the polarization separation element 51 and converts one of the two types of polarized light into the other type of polarized light. Light is incident on the polarization conversion element 50 in a manner that a major light beam (central axis) of the light is nearly parallel to a system optical axis AL.
The polarization separation element 51 includes a light incident face, a light emitting face, a plurality of glass materials 51a as light transmitting base materials, a plurality of polarization separation films 51b (shown in a solid line in the figure), and a plurality of reflection films 51c (shown in a dashed line in the figure). The light incident face is nearly orthogonal to the system optical axis AL, and the light emitting face is nearly parallel to the light incident face. The glass materials 51a are sequentially bonded on a plurality of inclined planes forming a predetermined angle with respect to the light incident face and the light emitting face. The polarization separation films 51b and the reflection films 51c are alternately formed on the plurality of inclined planes.
The predetermined angle of the plurality of inclined planes with respect to the light incident face (the light emitting face) is commonly 45°, and the glass materials 51a have a columnar shape extending in z axis direction and have a cross-sectional shape of an approximate parallelogram in x-y axis direction.
The polarization separation films 51b are composed of a dielectric multilayer film. The polarization separation films 51b separate a bundle of rays of incident light (including s-polarized light and p-polarized light) into a partial bundle of rays of s-polarized light (s-polarized light) and a partial bundle of rays of p-polarized light (p-polarized light), and reflects the s-polarized light and transmits the p-polarized light. On the other hand, the reflection films 51c are composed of a dielectric multilayer film or a metal film, and have a function of reflecting the s-polarized light being incident thereon. The half λ retardation plates 52 are provided in regions, through which the p-polarized light that transmits through the polarization separation film 51b passes, of the light emitting face of the glass materials 51 in a manner being arranged in a lattice. The half λ retardation plates 52 have a function of converting the p-polarized light being incident thereon into s-polarized light of which a polarization direction is orthogonal to that of the p-polarized light.
The polarization conversion element 50 is used with a light shielding plate 60 (shown in a dashed two dotted line in FIG. 22B) disposed on the light incident face thereof in order to make light that is incident thereon incident only on the polarization separation films 51b without making the light incident on the reflection films 51c. As the light shielding plate 60, a metal plate having a light shielding characteristic such as an aluminum plate and provided with openings 60a and light shielding parts 60b may be used.
In the polarization conversion element 50 structured as above, as shown in FIG. 22B, light (including s-polarized light and p-polarized light) incident on the glass materials 51a of the polarization separation element 51 through the openings 60a of the light shielding plate 60 is separated into two partial bundles of rays of the s-polarized light and the p-polarized light at the polarization separation films 51b. Then the s-polarized light obtained by separating the incident light at the polarization separation films 51b is reflected at the reflection films 51c, and the p-polarized light obtained by the separation transmits through the polarization separation films 51b so as to be converted into s-polarized light at the half λ retardation plate 52. That is, the s-polarized light that is a single type of polarized light obtained by the conversion in the polarization conversion element 50 is emitted nearly parallel to the system optical axis from the polarization conversion element 50.
JP-A-10-90520, as a first example, discloses the following manufacturing method in order to obtain a polarization conversion element of which a positional relation between light transmitting base materials is accurately set. The method includes a step of forming a polarization separation film on a first surface of a first light transmitting plate having the first surface and a second surface that are nearly parallel to each other; a step of forming a reflection film on the second surface; a step of alternately bonding a plurality of first light transmitting plates provided with the polarization separation film and the reflection film and a plurality of second light transmitting plates having two surfaces that are nearly parallel to each other; and a step of cutting the light transmitting plates obtained by alternately bonding the first plates and the second plates by a predetermined angle with respect to the first and second surfaces so as to produce optical blocks having a light incident face and a light emitting face that are nearly parallel to each other.
With respect to the common polarization conversion element mentioned above, JP-A-2004-170550, as a second example, discloses the following polarization conversion element. The polarization conversion element includes an incident face on which light is incident, a polarization separation face, a transmitting light emitting face, a reflection face, and a reflected light emitting face. The polarization separation face is disposed at an angle of approximately 45° with respect to the incident face. The transmitting light emitting face is provided to a position opposed to the incident face so as to be nearly parallel to the incident face, being an emitting part of transmitting light through the polarization separation face. The reflection face is disposed nearly parallel to the polarization separation face and further reflects a component reflected by the polarization separation face. The reflected light emitting face is disposed nearly parallel to the incident face and emits a reflected component. In the polarization conversion element, a region surrounded by the incident face, the polarization separation face, the reflection face, and the reflected light emitting face is made of transparent medium and a region which is opposite to the region for the transparent medium through the reflection face is composed of air.
In the polarization conversion element of the second example, the transparent medium having a parallelogram cross-section and surrounded by the incident face, the polarization separation face, the reflection face, and the reflected light emitting face and a transparent medium having a rectangular triangle cross-section and surrounded by the polarization separation face, the transmitting light emitting face, and a face orthogonal to the incident face are bonded through the polarization separation face. Then a plurality of polarization conversion elements structured as this are aligned on one surface of a lens array so as to allow the incident face to be a light converging part of the lens array. Alternatively, the plurality of polarization conversion elements are bonded and fixed on one surface of the lens array in an aligning manner.
Therefore, individual polarization conversion element totally reflects incident light incident on the reflection face thereof without providing an expensive reflection film to the reflection face, being able to reduce reflection loss by the reflection face. However, since the region opposite to the region for the transparent medium having the parallelogram cross-section is composed of air, an air region becomes a cavity having a rectangular triangle shape when a plurality of polarization conversion elements are aligned. Accordingly, in order to accurately align incident faces of the plurality of polarization conversion elements on one surface of the lens array by a predetermined interval, the manufacturing process becomes complex and a large number of steps are required.